As the development of the integrated circuit is now mushrooming, the polysilicon thin film transistor grown by the low pressure chemical vapor deposition (LPCVD) is extensively employed in the application of liquid crystal display (LCD) and high-density static random access memory. As an example, the active-matrix LCD (AM-LCD) which is the most dominant LCD product adopts the thin film transistor (TFT) as the display unit. Accordingly, the AM-LCD is power-saving and is able to display both the grayscale and the color. However, because there are a great deal of traps at the grain boundary in the polysilicon thin film, the polysilicon thin film transistor has larger threshold voltage and worse subthreshold swing. It has been reported that the plasma hydrogenation process is an efficient method to passivate these trap states. Nevertheless, the UV emitted from the discharged plasma glow will damage the boundary of the polysilicon/silicon oxide. Moreover, the plasma hydrogenation process brings about a low efficiency in passivating the tail state traps thereby limiting its applications. In conclusion, the conventional fabrication process for a polysilicon thin film transistor has the following shortcomings:
1. It is inefficient to passivate the tail state traps if the plasma hydrogenation process is combined into the conventional fabrication process for fabricating the polysilicon thin film transistor.
2. It will somewhat damage the boundary of the polysilicon/silicon oxide if the plasma hydrogenation process is combined into the conventional fabrication process for fabricating the polysilicon thin film transistor.
3. There is another approach for improving the characteristics of the polysilicon thin film transistor, that is, the channel oxidation process. The channel oxidation process provides a better efficiency for passivating the tail state traps than the plasma hydrogenation process does, and can prevent the damage caused by the UV emitted from the discharged plasma glow during the plasma hydrogenation process. Nevertheless, the application of channel oxidation will be limited as the gate oxide is getting thick.
Therefore, it is tried by the applicant to minimize the damage caused by the UV emitted from the discharged plasma glow and increase the efficiency for passivating the tail state traps.